Treatment method and process for valve seat

ABSTRACT

A method of treating a valve body is disclosed herein. The method includes using a medium carbon steel, and treating the valve body to a carburizing heating step, an intermediate annealing step, a hardening step, and a tempering step. These steps provide a durable valve seat surface for the valve body such that the valve seat can withstand the conditions that involve high friction or otherwise intense operating conditions, such as oil and gas applications.

FIELD OF INVENTION

The present disclosure relates to a process for treating a valve seat of a valve body.

BACKGROUND

Valve bodies are used in a wide range of applications and industries. In one application, valve bodies are used in oil and gas industries. These types of valve bodies include valve seats that must be especially durable due to excessive wear requirements. One known approach to address these issues is to use bainitically through hardened steels for these valve seats. However, this solution has disadvantages because these materials have a low fracture toughness due to the high carbon content throughout the entire valve seat. This can result in small microfractures that eventually lead to macroscopic cracks.

Another solution is case carburization on case hardening steels, which is achieved via a heat treatment. This results in increased core toughness due to a relatively low carbon content. Using this approach, the residual compressive stress is improved relative to bainitic steels, but there is a lack of carbides at the surface to counteract wear. Additionally, after wear begins, then the carbon content is reduced, which reduces the hardness. This ultimately leads to cracking after excessive wear.

Accordingly, it would be desirable to provide an improved method of producing and treating a valve seat that avoids the issues encountered by these known techniques.

SUMMARY

A method for treating a valve body is disclosed herein. The method includes a first heating step at a first predetermined temperature for a first predetermined period such that a valve seat surface of the valve body has a carbon content of 0.9%-1.4% up to a predetermined depth from an outer surface of the valve body. The method includes a second heating step at a second predetermined temperature for a second predetermined period, and the second predetermined temperature is less than the first predetermined temperature. The method includes a third heating step at a third predetermined temperature for a third predetermined period, and the third predetermined temperature is greater than the second predetermined temperature. A fourth heating step occurs at a fourth predetermined temperature for a fourth predetermined period, and the fourth predetermined temperature is less than the third predetermined temperature.

In one aspect, the predetermined depth is at least 0.3 mm. One of ordinary skill in the art would understand that this depth value can vary.

The valve body can be formed from steel having a carbon content of at least 0.28%.

In one aspect, carbides are precipitated in the valve body during the second heating step. During the third heating step, these carbides remain mostly unchanged. A total carbon content at surface can remain unchanged as well. In one aspect, the total carbon content at the surface be 0.9%-1.4%. In one aspect, 0.65%-0.9% carbon content is present in the matrix phase (i.e. the phase surrounding carbides). Stated differently, during the third heating step, carbon is already concentrated at surface in small carbides (i.e. C-content 100%) and solved in the surrounding austenitic matrix phase (i.e. C-content 0.65-0.9%).

The method can further include quenching the valve body after the third heating step. Other cooling or quenching steps can be carried out between the other heating steps as well. In another aspect, the third heating step occurs directly after the second heating step.

In one, the valve body includes the following: 0.15%-0.36% carbon; silicon; 0.60%-1.80% manganese; 0.80%-2.30% chromium; 0-0.50% nickel; 0.20-0.90% molybdenum; 0-0.6% vanadium; 0.015%-0.050% aluminum; 0-0.10% sulphur; 0-0.025% phosphor; 0-0.003% titanium; 0.005%-0.015% nitrogen; 0-12 ppm oxygen; 0-0.0035% calcium; and 0-0.25% copper.

In one aspect, the first predetermined temperature can be 940° C.-980° C., the second predetermined temperature can be 600° C.-660° C., the third predetermined temperature can be 800° C.-830° C., and the fourth predetermined temperature can be 100° C.-250° C.

Additional embodiments are disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing Summary and the following Detailed Description will be better understood when read in conjunction with the appended drawings, which illustrate a preferred embodiment of the disclosure. In the drawings:

FIG. 1 is a cross-sectional view of a valve body according to one aspect.

FIG. 2A is a microscopic image showing carbides in a valve seat according to one aspect of this disclosure.

FIG. 2B is another microscopic image showing carbides in a valve seat according to another aspect of this disclosure.

FIG. 3 is a chart illustrating the carbon content and hardness of a valve seat relative to a depth from an outer surface of the valve seat.

FIG. 4A is a chart illustrating steps for treating a valve seat relative to the respective time (t; x-axis) and temperature (T; y-axis) for each step.

FIG. 4B is a flow chart illustrating the steps for treating a valve seat.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain terminology is used in the following description for convenience only and is not limiting. “Axially” refers to a direction along an axis (X) of an assembly. “Radially” refers to a direction inward and outward from the axis (X) of the assembly. “Circumferentially” refers to a direction extending along a curve or circumference of a respective element relative to the axis (X) of the assembly.

A reference to a list of items that are cited as “at least one of a, b, or c” (where a, b, and c represent the items being listed) means any single one of the items a, b, or c, or combinations thereof. The terminology includes the words specifically noted above, derivatives thereof and words of similar import.

FIG. 1 illustrates a valve body 10 according to one aspect. The valve body 10 can have various configurations, as one of ordinary skill in the art would appreciate from this disclosure. A valve seat 12 is defined on the valve body 10. In one aspect, the valve seat 12 is immersed in slurry during operation.

In one aspect, the valve body 10 is formed from supercarburized medium carbon steel. The valve body 10 is formed from a base or raw material. In one aspect, the raw material for the valve body 10 can include the following elements:

-   -   0.15%-0.36% carbon;     -   0.20%-0.60% silicon     -   0.60%-1.80% manganese;     -   0.80%-2.30% chromium;     -   0-0.50% nickel;     -   0.20-0.90% molybdenum;     -   0-0.6% vanadium;     -   0.015%-0.050% aluminum;     -   0-0.010% sulphur;     -   0-0.025% phosphor;     -   0-0.003% titanium;     -   0.005%-0.015% nitrogen;     -   0-12 ppm oxygen;     -   0-0.0035% calcium; and     -   0-0.25% copper.

A remainder, i.e. the rest of the percentage composition not accounted for from the values above, of the valve body 10 is formed from iron and trace elements.

One of ordinary skill in the art would understand that various other elements could be included. For example, trace elements, such as antimony, tin, arsenic, or other elements.

A method 400 of treating a valve body is disclosed herein and specifically illustrated in FIGS. 4A and 4B. As shown by step 405, the valve seat is initially subjected to a high heat carburization process, which occurs at a first predetermined temperature. This step is also referred to as a first heating step or carburization step herein. During this process, the temperature (i.e. first predetermined temperature) can be held between 900° C. and 1000° C. for a predetermined period. In another aspect, the temperature is held between 940° C. and 980° C. The predetermined period for this step can vary, and can be at least 10 hours and no more than 40 hours, in one aspect. One of ordinary skill in the art would understand that is predetermined period can be less than 10 hours and more than 40 hours. The duration of the period can be used to determine carburization depth, but not carbon content at surface. The specific temperature and predetermined period can vary depending on the particular requirements for a valve seat, as well as its geometry and other characteristics. After this step, a surface content of at least 0.7% carbon and no more than 1.6% carbon is provided on the surface of the valve seat. In one aspect, the carbon content has a constant value up to a depth of at least maximum stock removal plus 0.3 mm. Carbon content at the surface of the component has a carbon gradient. After heat treatment, the component is typically hard machined (e.g., grinded, honed). Stock removal of approximately 0.3 mm can be typical for valve seat components. In other words, up to a depth of 0.3 mm from the surface material can be mechanically removed after heat treatment. After hard machining, at least 0.9% of carbon can be available at the surface. A value of 0.9% carbon is considered a “lower carbon” content for carbide formation. One of ordinary skill in the art would understand that the carbon content decreases as the distance from the surface increases. In one aspect, 0.7% of the carbon from this step is used to create a hardness in the martensite. In one aspect, this hardness can be approximately 67 HRC after quenching and before tempering. Any carbon content exceeding the 0.7% value can form soft austenite if the steel were to be quenched and prior to tempering. However, in another aspect, this carbon content can also be used to create fine and precipitated carbides, as disclosed in further detail below, and as shown in FIGS. 2A and 2B which show the surface of the valve seat after treatment according to the steps disclosed herein.

Carbon content exceeding 0.7% forms soft retained austenite exceeding 20% of the total volume of material after the first quenching and after the first heating step. This excessive retained austenite dissociates during the second heating step, which results in the formation of fine and homogeneously distributed carbides. These carbides remain mostly unchanged during the third heating step. Carbon solved in the matrix phase (i.e. austenite) during the third heating step is used to create martensite after quenching. Furthermore, retained austenite after third heating step is available as well.

After step 405, the valve seat can be cooled to room temperature (i.e. ambient temperature, approximately 20° C.) as shown by step 410. In one aspect, this cooling step can be performed via quenching.

Next, during step 415, the valve seat is heated to a second predetermined temperature. In one aspect, the second predetermined temperature is less than the first predetermined temperature. In one aspect, the second predetermined temperature can be at least 550° C. and no greater than 700° C. In another aspect, the second predetermined temperature is at least 600° C. and no greater than 660° C. This heating step is also referred to as a second heating step or intermediate annealing step herein. During this step, the valve seat can be heated for a second predetermined period of at least 1 hour, and no more than twelve hours. In one aspect, this second heating step is between three hours and ten hours. This second heating step is primarily directed to precipitating carbon to carbides. In one aspect, carbon is precipitated in the form of M3C, wherein M can be predominantly iron. As used in this context, the term predominantly means over 50%. As used in this context, M3C refers to a ratio of metallic elements to carbon in carbide. In this instance, M3C can refer to three atoms of metals “M” (such as Fe, Mn, Cr, Mo, etc.) to one atom of carbon “C.” One of ordinary skill in the art would understand that carbon can be precipitated in other forms for carbides, such as M23C6, M7C3, M6C, where M refers to the sum of carbide forming metallic elements. In one aspect, M can also include at least one of chromium or molybdenum. In one aspect, a percentage of chromium can be up to 25%, and a percentage of molybdenum can be up to 25%.

Following step 415, the valve seat can be cooled to room temperature, in one aspect, as shown by step 420. In another aspect, step 420 can be omitted and the valve seat can continue to a subsequent heating step directly from the second heating step.

As shown by step 425, the valve seat subsequently undergoes a third heating step, i.e. a hardening step, in which the valve seat is heated to a third predetermined temperature for a third predetermined period. The third predetermined temperature can be at least 800° C. and preferably less than 900° C. The third predetermined temperature can be at least 800° C. and less than 830° C. The third predetermined period can be between 0.5 hours to 2.0 hours. During this step carbon content up to 0.9% is completely solved in austenite. Carbon content exceeding is presented in form of fine carbides up to a depth of 0.5 mm-4.0 mm, which is dependent upon the carburizing time in first heating step. This step causes a limited amount of retained austenite to be present in the valve seat. As used in this context, the term “limited amount” refers to approximately 20%-45%. A carbon content of to 0.90% can be formed in an austenitic matrix phase on the outer surface of the valve body during the third heating step.

Following step 425, the rest of the carbon in the valve body remains bound as carbides, with a hardness of at least 1000 HV and no greater than 3000 HV up to a depth of 0.5 mm-4.0 mm, depending on carburizing time in first heating step. This configuration provides a hardness that is significantly greater than the steel matrix, which increases wear resistance. Carbides with high hardness generally act as hard particles embedded in a soft matrix phase. This configuration generally increases total hardness of the component and makes its material more resistant to wear.

The valve body can then be cooled or quenched during step 430. In one aspect, this cooling or quenching can be performed using hot salt, which can include up to 2.0% water. In one aspect, the hot salt solution includes up to 1.5% water. Quenching is performed at a fourth predetermined temperature. In one aspect, the fourth predetermined temperature is at least 100° C. and no more than 250° C. In one aspect, the fourth predetermined temperature is 170° C.-220° C. Quenching can be performed for a fourth predetermined period, which can be less than one hour, and preferably 20 minutes-40 minutes. Alternatively, an oil bath can be used for quenching. The oil bath can be held at a temperature of at least 20° C. and no more than 120° C. In yet another embodiment, a polymer bath can be used having a temperature of at least 20° C. and no more than 70° C. In one aspect, quenching in a salt bath can be performed at the relatively lowest temperature and produce the lowest stress gradients in the component during quenching, which minimizes the danger of cracks.

In one aspect, a cryogenic treatment can be performed prior to a final tempering step. The cryogenic treatment can include cooling the valve body to at least −20° C. During cryogenic treatment of a component, the soft retained austenite can be transformed to hard martensite, which increases total hardness of the material.

Finally, a tempering step is performed, as shown by step 435. During tempering, a fifth predetermined temperature can be used that is at least 100° C. and no more than 250° C. This tempering step can be performed for a fifth predetermined period. In one aspect, the fifth predetermined period can be at least one hour and no greater than four hours.

In one aspect, the valve body 10 disclosed herein is formed from a medium carbon steel, i.e. 0.3% carbon. This provides higher structural strength as compared to previously known carburizing steels, i.e. steels with less than 0.25% carbon, while still providing for improved fracture resistance and behavior as compared to through hardened steel grades, such as 100C6.

As shown in FIG. 3 , the valve body formed according to the processes disclosed herein has a carbon content of approximately 1.2%-1.4% up to a depth of approximately 0.5 mm. The carbon content is 1.4%-0.8% up to a depth of approximately 2.0 mm. The hardness value HV ranges from approximately 800 HV at the surface to approximately 750 HV at a depth of 2.0 mm. In one aspect, the hardness value HV ranges from 750-850 HV at the surface to approximately 700 -800 HV at a depth of 2.0 mm

Using the process and method disclosed herein, a valve body having a valve seat with improved durability is provided. The valve seat has improved resistance to cracking and resistance to wear that typically occurs in valve bodies used in oil and gas applications due to exposure to slurries and other abrasive fluids or substances.

Having thus described the present disclosure in detail, it is to be appreciated and will be apparent to those skilled in the art that many physical changes, only a few of which are exemplified in the detailed description of the invention, could be made without altering the inventive concepts and principles embodied therein.

It is also to be appreciated that numerous embodiments incorporating only part of the preferred embodiment are possible which do not alter, with respect to those parts, the inventive concepts and principles embodied therein.

The present embodiment and optional configurations are therefore to be considered in all respects as exemplary and/or illustrative and not restrictive, the scope of the embodiments being indicated by the appended claims rather than by the foregoing description, and all alternate embodiments and changes to this embodiment which come within the meaning and range of equivalency of said claims are therefore to be embraced therein.

LOG OF REFERENCE NUMERALS

-   -   Valve body 10     -   Valve seat 12 

1. A method for treating a valve body, the method comprising: a first heating step at a first predetermined temperature for a first predetermined period such that a valve seat surface of the valve body has a carbon content of 0.9%-1.4% up to a predetermined depth from an outer surface of the valve body; a first cooling step performed via quenching; a second heating step at a second predetermined temperature for a second predetermined period, wherein the second predetermined temperature is less than the first predetermined temperature; a third heating step at a third predetermined temperature for a third predetermined period, wherein the third predetermined temperature is greater than the second predetermined temperature; and a fourth heating step at a fourth predetermined temperature for a fourth predetermined period, wherein the fourth predetermined temperature is less than the third predetermined temperature.
 2. The method according to claim 1, wherein the predetermined depth is at least 0.3 mm.
 3. The method according to claim 1, wherein the valve body is formed from steel having a carbon content of at least 0.28%.
 4. The method according to claim 1, wherein carbides are precipitated in the valve body during the second heating step, and a carbon content of 0.65% to 0.90% is formed in an austenitic matrix phase on the outer surface of the valve body during the third heating step.
 5. The method according to claim 4, further comprising quenching the valve body after the third heating step.
 6. The method according to claim 1, wherein the third heating step occurs directly after the second heating step.
 7. The method according to claim 1, further comprising cooling the valve body to room temperature between the second and third heating steps.
 8. The method according to claim 1, wherein the valve body comprises: 0.15%-0.36% carbon; 0.20%-0.60% silicon; 0.60%-1.80% manganese; 0.80%-2.30% chromium; 0-0.50% nickel; 0.20%-0.90% molybdenum; 0-0.6% vanadium; 0.015%-0.050% aluminum; 0-0.010% sulphur; 0-0.025% phosphor; 0-0.003% titanium; 0.005%-0.015% nitrogen; 0-12 ppm oxygen; 0-0.0035% calcium; 0-0.25% copper; and a remainder of a composition of the valve body is formed from iron.
 9. The method according to claim 1, wherein the second predetermined temperature is 600° C.-660° C.
 10. The method according to claim 1, wherein the third predetermined temperature is 800° C.-830° C.
 11. The method according to claim 1, wherein the first predetermined temperature is 940° C.-980° C.
 12. The method according to claim 1, wherein the fourth predetermined temperature is 100° C.-250° C.
 13. The method according to claim 1, wherein the first predetermined temperature is 940° C.-980° C., the second predetermined temperature is 600° C.-660° C., and the third predetermined temperature is 800° C.-830° C. 14-20. (canceled) 